ND Filter
Many small form factor cameras such as the IXUS/SD series only have one real aperture setting, which is commonly f/2.8 at wide-angle (i.e. when the camera is not zoomed in at all). The lens in all point-and-shoot cameras (and some D/SLR zoom lenses) has a variable maximum aperture value (lowest f number), which means Canon will list the camera with zoom specs 6.2 - 18.6mm and aperture f/2.8 - 4.9. In this case, the maximum aperture will be f/2.8 when the camera is at its widest focal length (6.2mm) and increases as you zoom in until the telephoto setting (of 18.6mm) is reached with an f-stop maximum value of f/4.9. That is, when zoomed in at 18.6mm the camera is at f/4.9 and cannot go any lower (i.e. to f/2.8). This is why there are different aperture values when zooming in, despite these cameras having a single fixed aperture. How the ND filter in these cameras work When your camera detects a really bright scene, it will swing the ND filter in. This in effect makes the scene look darker and will effect the exposure (but not depth of field as a real aperture would). With the ND filter in, the camera can select a slower shutter speed without overexposing the entire scene. Also, with the ND filter swung in, the camera will capture an image with an aperture of f/8 - This is a fake aperture value caused by the ND filter swinging in. With CHDK the ND filter out can be kept out, but a faster shutter speed to compensate as to be selected. In the CHDK menu this operation can be found at: Extra Photo Operations -> ND filter state Just select one of the options from there: Off/In/Out * 'Off' means the camera is in control of the ND filter state. * 'In' means the ND filter is swung in. * 'Out' means the ND filter is not in. Notes: * Setting the ND position in a script takes effect just before the actual shot is taken and after the exposure is set; * Not all cameras have an ND filter; all cameras of the Ixus series have a ND Filter instead of a real aperture, also some other ones, e.g. the A450, A460, A530, A550, A560, A1000, TX1. * Some higher end cameras have both an ND filter and a real aperture, e.g the cameras of the G series. ND Filter Scripting Functions get_nd_present() - return status indicates whether camera has an ND filter and/or a diaphragm set_nd_filter() - 0=auto, 1=in, 2=out An example HDR script using the ND filter: http://chdk.wikia.com/wiki/ND_Filter_HDR_Script. List of cameras with ND filter (* Per platform/(model)/platform_camera.h - CAM_HAS_ND_FILTER) (** These cameras almost certainly do not actually have an ND filter, but in current CHDK releases have CAM_HAS_ND_FILTER and respond to the set_ND_filter function by changing the aperture, see https://chdk.setepontos.com/index.php?topic=13228.40) Resources A disassembled IXUS800 (SD700) with photos of lens and ND Filter - http://chdk.setepontos.com/index.php/topic,548.0.html External Resources * Learn more about Neutral Density filters - http://en.wikipedia.org/wiki/ND_filter * Learn more about Minimum/Maximum Apertures - http://en.wikipedia.org/wiki/Aperture#Maximum_and_minimum_apertures * Understanding Camera Lenses - http://www.cambridgeincolour.com/tutorials/camera-lenses.htm Category:Help